Method for applying a hot melt ink to a substrate

ABSTRACT

The present invention provides a process for applying a thermoplastic image forming composition as a series of discrete droplets from a non-contact ink jet printing apparatus to form separate drops on a substrate moving relative to the apparatus, characterized in that the molten composition is thermally stable at the temperature of application and is applied at a temperature in excess of 100° C. 
     The invention can be used to apply the molten composition to a variety of substrates using on-demand or continuous non-contact ink jet application techniques. However, the invention is of especial use in the application of thermoplastic inks to non-porous substrates using an on-demand ink jet printer.

The present invention relates to a method for applying a composition toa substrate and to a composition for use therein, notably to a methodfor applying thermoplastic inks and to a novel thermoplastic inkcomposition.

It has been proposed to apply inks through an ink jet printing machinewhere the inks are in the form of wax based compositions which areapplied molten through the nozzle of the printer. In order to reduceproblems which would be expected in attempting to operate at elevatedtemperatures (for example degradation of the composition), the priorproposals have required the use of comparatively low melting pointcompositions and low temperatures of operation. Thus, for example, inU.S. Pat. No. 3,653,932 the composition is required to have a meltingpoint which does not exceed 51° C. and contains a didodecyl sebacatewhich is a highly viscous material. In order to overcome the problemsassociated with that formulation, U.S. Pat. No. 4,390,369 proposes theuse of a composition which comprises a natural wax and has a meltingpoint below about 75° C. The use of natural waxes is also proposed inEuropean application No. 097823, where the composition comprises amixture of paraffin wax and stearic acid. However, such compositions donot adhere satisfactorily to plastics substrates, suffer from smudging,and problems are encountered due to the high viscosity of the componentswhere synthetic mamterials are used.

Contrary to the teaching of the prior proposals, we have found that itis advantageous to operate a hot melt ink jet printer at a temperaturein excess of 100° C. At such elevated temperatures, the waxes of theearlier proposals would often decompose whereas in the present inventionthey result in an image of improved definition, which resists smudgingand has improved adhesion to plastics substrates, and also reduces theproblems associated with the use of high viscosity materials.

U.S. Pat. No. 3,369,253 discloses a number of compositions for use in apen type chart recorder in which ink flows continuously from a nozzleonto a moving substrate to draw lines thereon. There is direct contactbetween the nozzle and the substrate via the ink composition and thecomposition must have a sufficiently high surface tension at thetemperature of operation to be pulled from the nozzle as a continuousstream and not as a series of individual droplets. Such requirements aretotally the reverse of what is required in an ink jet printer wherethere is no direct contact between the printer and the substrate and theink issues as a series of discrete droplets. Furthermore, in the U.S.patent the substrate has to be of a specified type with a mandatorysurface layer in order that the molten composition should be not causeappreciable dimensional changes in the substrate. The need for a specialsurface on the substrate severely limits the possible fields of use ofthis technique.

Accordingly, the present invention provides a process for applying athermoplastic composition through a non-contact ink jet printingapparatus as a series of discrete droplets onto a substrate movingrelative to the apparatus, characterised in that the molten compositionis thermally stable at the temperature of application and is applied ata temperature in excess of 100° C.

The invention can be used to apply the molten composition to a varietyof substrates using on-demand or continuous non-contact ink jetapplication techniques. However, the invention is of especial use in theapplication of thermoplastic inks to a substrate using an on-demand inkjet printer.

In an on-demand ink jet printer, ink is fed under pressure, typically0.5 to 20 psi gauge, though higher pressures may be used if desired,from a reservoir to a series of nozzles via valve means which controlthe flow of the ink through each nozzle. The valve means is typically anelectro-magnetically actuated valve, notably a solenoid valve. The inkis discharged through the nozzles as discrete droplets in the desiredsequence to form the required image on the substrate. Usually, thenozzles are arranged in one or more series transversely to the line ofmovement of the substrate. Typically, such printers have quick actingvalves with an operating cycle time of from 1 to 5 milliseconds feedingnozzles with orifices having bore diameters of from 0.01 to 0.45 mms andan internal bore length to diameter ratio of from 3:1 to 1:2, notablyfrom 2:1 to 1:1.

The thermoplastic compositions for present use comprise one or moreimage forming components, preferably oil miscible or soluble, in afusible carrier medium. The image forming material can be one whichforms a visual image on the substrate, e.g. it can be a dyestuff, or onewhich is detected by other means, e.g. it can be a magnetic material tobe scanned by a suitable reader, or a fluorescent material, e.g. onewhich is detected by an ultra-violet or other radiation scanner. Forconvenience, the present invention will be described in terms of acomposition containing a visually detectable dyestuff.

The composition for present use is preferably a solution or emulsion ofthe dyestuff in the carrier medium, i.e. it is substantially free fromparticles which might block the nozzles through which the composition isto be discharged. If necessary, the composition can contain a co-solventor a bridging fluid to aid formation of a substantially homogeneouscomposition.

The compositions comprise a fusible carrier medium which is renderedmolten at the temperatures encountered in the method of the invention.The fusible carrier media for present use are thermally stable at thetemperature of application and satisfy the surface tension and viscosityrequirements of the ink jet printer they are to be applied through. Ingeneral, the carrier should not be thermally degraded or decomposed attemperatures of up to 160° C. and should have a viscosity of less than120 Cps at the temperature of application. They should have a surfacetension which is sufficiently low at the operating temperature for theink to form discrete droplets rather than a continuous jet which forms abridge bewteen the nozzle and the substrate. In view of the difficultiesin measuring surface tensions at elevated temperatures, the mostconvenient test of suitability for present use is to run the compositionthrough the ink jet printer in which it is to be used to ascertainwhether it forms a continuous jet or discrete droplets at the operatingtemperature. In many cases measurement of surface tension at 25° C. willgive a prima facie indication as to the suitability or otherwise of acomposition for present use. The surface tension is determined byestablishing whether a sample of the composition is wet or not by aseries of fluids of known surface tension. Where the solid compositionis wet, it has a higher surface tension than the fluid, where the fluidforms a stable droplet on the surface of the composition the compositionhas a lower surface tension. In general, where the composition has asurface tension of 50 dynes per cm or less in the above test, it will besuitable for present use.

The carrier media for present use can be selected from amonst naturalwaxes having the desired porperties, but we prefer to use syntheticmaterials. We have found that microcrystalline waxes, notably thesynthetic forms of such waxes, and/or hydrocarbon resins provideparticularly advantageous carrier media, in that they can provide highlymobile molten compositions with reduced risk of degradation at highoperating temperatures, which will typically be at from 110° to 160° C.,notably at from 125° to 150° C. Moreover, where the composition has asoftening point greater than 60° C., the rapid cooling of the dropletsof the composition as they strike the substrate and the partial fusingof the substrate below the hot droplet in the case of plasticssubstrates gives an image which resists smudging.

The invention therefrom also provides a fusible ink compositioncomprising one or more oil soluble or miscible indicator materials,notably a dyestuff, in a carrier medium comprising a syntheticmicrocrystalline wax and/or a hydrocarbon resin, the composition havinga viscosity of less than 120 Cps, a surface tension has determined bythe test method described above) of less than 50 dynes per cm at 25° C.and a softening point of from 60° to 110° C.

The microcrystalline waxes for present use can be selected from a widerange of such waxes which are available commercially. Typically, the waxwill be a synthetic hydrocarbon wax obtained from the processing ofpetroleum or naphthas, notably the naphthenic, polyethylene orpolypropylene waxes which have softening points in the range 60° to 100°C., notably 70° to 90° C. Other suitable waxes include those obtained bythe Fischer Tropsch process, typically those comprising long chainlinear paraffins with molecular weights of from 300 to 1500 andsoftening points in the range 80° to 100° C. Preferred syntheticmicrocrystalline waxes for present use include Slack Wax and thepolyethylene waxes obtained from the lighter fractions of the crackingof naphtha and petroleum. If desired, the microcrystalline wax may beused in the form of a derivative thereof, e.g. as an oxidised ormaleinised derivative.

The hydrocarbon resins for present use are preferably crystallineresins, notably C₅ to C₉ chain length aliphatic waxes or polyolefinswith softening points greater than 90° C., typically 90° to 110° C., andmelting points greater than 110° C., preferably in the range 140° to160° C. Typically, the polyethylene resins will have a mean molecularweight of less than 1500, e.g. 500 to 1200 and an acid number of lessthan 1 (expressed as mg KOH/g). Suitable hydrocarbon resins for presentuse thus include polyolefins, notably polyethylene, polypropylene orpolybutylene; C₅ to C₉ chain aliphatic resins, e.g. those obtained bythe steam cracking of naphthas; polyterpenes, notably wood rosins, talloil or balsam resins, which can be esterified or hydrogenated ifdesired; and aromatic compounds, e.g. styrenes such as methyl styrenes.

Whilst the hydrocarbon resin may often be a microcrystalline material,we have found that the use of a mixture of both an hydrocarbon resin anda synthetic microcrystalline wax as described above is of especialadvantage, since the hydrocarbon resin enhances the adhesion of the waxbased composition to non-porous substrates, notably to plastics sheetsubstrates. Furthermore, by varying the proportions of the resin and waxit is possible to tailor make the properties of the composition, e.g.the viscosity, to suit a wide range of operating conditions.

The compositions of the invention may contain other ingredients inaddition to the microcrystalline wax, resin and image forming material.Thus, the compositions can contain thermal and/or UV stabilisingmaterials to reduce the degradation of the ingredients of thecomposition; and minor proportions of one or more solvents orco-solvents for the ingredients to aid formulation of the composition asa substantially homogeneous mixture.

The compositions for present use preferably have a viscosity of from 2to 120, notably 3 to 20, Cps at the temperature of operation of theprinter, typically 120° to 160° C.; and a surface tension of less than40 dynes per cm, notably less than 30 dynes per cm, at 25° C. using thetest method described above.

It may be desired to include one or more viscosity and/or surfacetension modifying agents in the composition to achieve the desiredviscosity and/or surface tension at the actual operating temperaturesfor optimum operation of a particular printer. However, as indicatedabove, we have found that the viscosity and the surface tension areaffected by the relative proportions of the microcrystalline wax and thehydrocarbon resin in the composition and that the viscosity and surfacetension can often be adjusted to the desired values merely by varyingthe relative proportions of these two ingredients.

Where the compositions are to be applied using a continuous jet inkprinter, it is necessary tht the composition be one which can accept anelectrical charge. This is conveniently achieved by including one ormore ionic or polar materials in the composition, e.g. potassiumisothiocyanate. In order to reduce the risk of segregation of thesematerials from the composition, it may be desired to incorporate abridging compound, e.g. a wetting agent of the alkyl ether sulphate orsulphonate or of the alkyl benzene sulphonate type, into thecomposition. Typically, the composition for application through acontinuous jet printer will have a conductivity of at least 1000,preferably 1500 to 2500, microSiemens.

The compositions of the invention comprise the microcrystalline wax, thehydrocarbon resin and the image forming material in any suitableproportion having regard to the nature of the ingredients, the nature ofthe substrate it is to be applied to and the operating conditions underwhich it is applied. Typically, the composition comprises at least oneselected from a microcrystalline wax or an hydrocarbon resin, themicrocrystalline wax being present in up to 99.9%, notably from 40 to99.5%; the hydrocarbon resin being present in from 0 to 65%; notably 25to 55%; and the image forming material being present in from 0.05 to 5%,preferably 0.1 to 2%; all percentages being of the active material andby weight on the weight of the total composition.

A particularly preferred composition for present use comprises from 40to 99 parts by weight of a micro-crystalline polyethylene wax, from 0 to60 parts by weight of a polyethylene or styrene hydrocarbon resin ofmolecular weight from 500 to 1200 and 0.1 to 1.5 parts by weight of anoil soluble dyestuff.

The compositions of the invention can be made by any suitable technique,e.g. by melting the wax and/or resin components thereinto.

The compositions can be put up in the form of powders or granules byspraying the molten mixtures of the components into a void vessel.Alternatively, they can be extruded through a suitable die to formmoulded plugs of the composition for insertion into a suitable shapedreceptacle in the printer for melting and use.

As stated above, the compositions of the invention are applied to asubstrate by passing them through the nozzle or array of nozzles of anink jet printer apparatus at an elevated temperature. The apparatus canbe of conventional design, except that those parts of the apparatusthrough which the molten composition is to flow are heated or insulatedso as to reduce the risk of the composition solidifying within theapparatus. Such heating can be achieved by any suitable means, e.g. byelectrical heating elements around the appropriate ducts or vessles orby infra red or other radiant heaters playing on the appartus.Typically, the apparatus heaters operated with the composition flowingtherethrough at temperatures of from 125° to 150° C., e.g. about 140° C.

The composition is fed to the apparatus in solid form, e.g. as chips orgranules or the shaped plugs described above, and is melted in asuitable vessel attached to or forming an integral part of the printerapparatus. If desired, the compositions can be held in a separate heatedreservoir and fed to one or more individual printers through heated orinsulated lines.

Surprisingly, the presence of the microcrystalline wax or hydrocarbonresin, notably the combination thereof, does not have the deleteriouseffect of radically increasing the viscosity of the composition as isthe case with the sebecate waxes proposed hitherto. The printingapparatus can therefore be operated under similar pressures and flowrates as with a conventional ink formulation and using nozzle orificesin the range 10 to 450 microns diameter.

The compositions of this invention can be applied to a wide range ofporous and non-porous substrates, e.g. paper, metal, wood, plastics orglass without the need to form any special surface layer on thesubstrate. However, the invention is of especial use in forming imageson non-porous materials, e.g. plastics, plastics coated materials, glassand metals. The high temperature of the composition as it is despositedon a plastics substrate causes enhanced adhesion of the composition dueto partial fusion with the substrate. In the case of porous substratesthe compositions penetrate into the substrate as they cool. In bothcases, the compositions of the invention solidify rapidly on thesubstrate to give a sharp image resistant to smudging.

The invention has been described above in terms of the application of anink-type composition. However, it can also be applied to adhesivecomposition, for example those containing polyacrylamide or similarpolymers. Whilst it is known to apply hot melt adhesives from a gun orother applicator which extrudes the molten adhesive onto a substrate, webelieve that it is novel to apply hot melt adhesives as droplets emittedfrom an ink jet printer where the droplets are emmitted at a frequencyof more than 100 drops per second from a nozzle.

The invention therefore also provides a method for applying a moltenadhesive composition to a substrate characterised in that the adhesivecomposition is dispensed at a temperature in excess of 100° C. through anozzle to form a series of droplets which are applied to the substrate,the droplets being formed at the nozzle at a rate of more than 100 persecond.

The invention will now be illustrated by the following Examples in whichall parts and percentages are given by weight:

EXAMPLE 1

An ink formulation was prepared by melting the microcrystallinepolyethylene wax commercially available under the trade name ShellMicrocrystalline wax MMP (54 parts) in a heated vessel fitted with astirrer. To the molten wax were added the oil soluble dyestuff CeresBlue (1 part) and 45 parts of the hydrocarbon resin sold under the tradename Escorez 5300. The resultant mixture had a softening point of 85° C.and a melting point of 90° C., both as determined by Brookfieldthermoset viscometer, and surface tension of less than 40 dynes per cmas determined by the test method described earlier.

The composition was fed to a drop-on-demand ink jet printer havingelectrical heating coils to maintain the ink reservoir, ink lines andprinting head at 140° C.±5° C. The reservoir was pressurised to 2 psigauge and the molten composition printed through the print head using a0.225 mms bore orifice to produce a series of separate droplets whichformed discrete dot images on a paper or polyethylene sheet targetplaced below the printing head. The dots were sharply defined, wellanchored to the substrate and resistant to smudging.

By way of contrast, when natural carnuba wax was used in place of themicrocrystalline wax and hydrocarbon resin, the composition had amelting point of 50° C. and began to degrade when held at 120° C. for 1hour, as evidenced by charring. When this composition was applied to apaper substrate at 80° C., the image was soft and susceptible tosmudging. Where a polyethylene sheet was used as the substrate, theimage did not adhere to the sheet and was readily wiped off.

EXAMPLE 2

The process of Example 1 was repeated with a range of different resinsand waxes being applied onto different target substrates includingglass, metal, polyethylene, polypropylene, PVC, polystyrene, wax andplastics coated paper and card. The compositions are set out below asformulations A, B and C and in all cases the compositions were appliedas in Example 1 and gave good dot images.

When the compositions were replaced by conventional sovent basedformulations or low temperature wax formulations and applied to anon-porous substrate, the resultant dots either did not dry rapidly andgave a runny image (in the case of a solvent ink) or were soft andreadily smudged (in the case of the low temperature waxes).

Formulation A

54.7% wood rosin hydrocarbon resin

45.0% low melting point microcrystalline polyethylene wax

0.3% oil soluble dyestuff

Formulation B

70.0% medium melting point microcrystalline polyethylene wax

29.5% commercially available C₅ -C₉ aliphatic hydrocarbon resin

0.5% oil soluble dyestuff

Formulation C

55% commercially available water white grade alpha methyl styrenehydrocarbon resin

44.9% of a commercially available mixture of microcrystalline andparaffin waxes

0.1% oil soluble dyestuff

I claim:
 1. A process for applying a thermoplastic composition to asubstrate, comprising the steps of:moving the substrate past anapplication station at which is located a non-contact ink jet printingapparatus; operating the non-contact ink jet printing apparatus to applythe composition to the moving substrate as a series of discretedroplets, the composition being applied at a temperature in the rangefrom 100° to 160° C., the composition being characterized as beingthermally stable at the temperature of application and having aviscosity of from 2 to 20 Centipoise at the temperature of application.2. A process as claimed in claim 1 characterized in that the compositionhas a surface tension of less than 50 dynes per cm at 25° C.
 3. Aprocess as claimed in claim 1, further comprising the steps of:providinga reservoir as a part of the non-contact ink jet printing apparatus atthe application station; holding the composition in the reservoir at theapplication temperature so that the composition is maintained in amolten state; feeding the composition from the reservoir under apressure of from 0.5 to 20 psi gauge to a series of nozzles fordischarging the composition onto the substrate as a series of droplets;and controlling the discharge of the composition from the nozzles byproviding valve means between the reservoir and nozzles.
 4. A process asclaimed in claim 3 characterised in that the valve has an operatingcycle time of from 1 to 5 milliseconds, the nozzle has an orifice borediameter of from 0.01 to 0.45 mms and an internal bore length todiameter ratio of from 3:1 to 1:2.
 5. A process according to claim 1characterised in that the thermoplastic composition comprises one ormore oil miscible or soluble image forming components in a fusiblecarrier medium.
 6. A process as claimed in claim 5 characterised in thatthe fusible carrier medium is thermally stable at temperatures of up to160° C., has a viscosity of less than 120 Cps at the temperature ofapplication and has a surface tension of less than 40 dynes per cm at25° C.
 7. A process as claimed in claim 6 characterised in that thefusible carrier medium comprises at leat one component selected from asynthetic microcrystalline wax, an hydrocarbon resin and mixturesthereof.
 8. A process as claimed in claim 7 characterised in that themicrocrystalline wax is selected from polyolefin or paraffin waxes withmolecular weights of from 300 to 1500 and softening points in the range80° to 110° C.
 9. A process as claimed n claim 7 characterised in thatthe hydrocarbon resin is selected from crystalline C₅ to C₉ chain lengthaliphatic resins and polyolefin resins with softening points greaterthan 90° C. and melting points greater than 110° C.
 10. A process asclaimed in claim 7 characterised in that the polyolefin resin has a meanmolecular weight of less than 1500 and an acid number of less than 1(expressed as mg KOH/g).
 11. A process as claimed in claim 6characterised in that the molten composition has a viscosity of from 3to 20 Cps at the operating temperature; and a surface tension of lessthan 40 dynes per cm at 25° C.
 12. A process as claimed in claim 6characterised in that the microcrystalline wax is present in from 40 to99.5%; the hydrocarbon resin in from 25 to 55% and the image formingmaterial in from 0.1 to 2%; all percentages being of the active materialand by weight on the weight of the total molten composition.
 13. Aprocess as claimed in claim 6 characterised in that the compositioncomprises from 40 to 99 parts by weight of a microcrystallinepolyethylene wax; from 0 to 60 parts by weight of an hydrocarbon resinselected from C₅ -C₉ aliphatic resins, a polyethylene resin and astyrene resin said latter two resins having a molecular weight of from500 to 1200; and from 0.1 to 1.5 parts by weight of an oil solubledyestuff.
 14. A process as claimed in claim 1 characterised in that theink jet printing apparatus is operated at a fluid temperature of from125° to 150° C.
 15. A method according to claim 1 for applying a moltenadhesive composition to a substrate characterised in that the adhesivecomposition is dispensed at a temperature in excess of 100° C. through anozzle to form a series of discrete droplets which are applied to thesubstrate, the droplets being formed at a rate of more than 100 dropletsper second.